This invention relates to an attachment for the cleaning sieve of a combine harvester and to a combine harvester including such an attachment.
Conventional combine harvesters generally include means for separating grain and chaff from straw of a harvested crop. In many cases this is carried out by a cylinder, concave and beater together with a plurality of straw walkers. The separated grain and chaff falls from the concave and straw walkers onto and is collected by a grain pan which extends beneath the general area of those parts. The grain pan is generally serrated so that a jerking reciprocating movement of the grain pan acts to transport the grain and chaff longitudinally of the pan toward the rear of the combine. From a discharge end of the grain pan, the chaff and grain falls generally downwardly onto a cleaning sieve or shoe which comprises an inclined surface which is perforated by a plurality of openings and is again reciprocated commonly with the pan so as to move the grain and chaff generally toward a rear discharge end of the sieve. Air is directed from a fan beneath the sieve through the openings so as to cause a separation of the lighter chaff from the heavier grain so that the chaff is thrown rearwardly over the discharge end of the sieve while the grain drops through the sieve onto a second cleaning sieve for further cleaning and collection.
The main effect which acts to cause the separation of the grain from the chaff is therefore that of an air separation using the air stream through the sieve. For this reason the perforations in the sieve are generally adjustable in size so as to increase or reduce the amount of air flow. This separation technique has become the standard for almost all combine harvester equipment but is not entirely satisfactory. Even in the best conditions under exact settings, grain can be lost in measurable quantities and this is accepted along with other losses which are conventional in farming.
However in cases where the amount of grain and chaff falling onto the sieve is either significantly increased or decreased from the optimum for which the settings have been chosen, the separation can seriously deteriorate. Thus if a reduced amount of grain and chaff is deposited upon the sieve, the air stream can be too great so that not only chaff but also some of the lighter grain can be ejected from the discharge end of the sieve.
In cases where the grain and chaff exceed the set amount, a mat of grain and chaff can form on the sieve since the airflow is generally too light so that the mat including significant quantities of grain discharged from the end of the sieve or even a serious blockage of the system can occur.
It is of course not possible to constantly reset the airflow in dependence upon variable crop amounts which can of course occur as the combine is moved from area to area depending upon soil conditions, ground contour and other factors.